FALSELY LOW RESULTS IN CA-125 DETERMINATION DUE TO ANTIIDIOTYPIC ANTIBODIES INDUCED BY INFUSION OF [I-131] F(AB')(2) FRAGMENTS OF THE OC125ANTIBODY

We investigated a one-step immunometric CA 125 assay, which employs ne w anti-CA 125 antibodies as capture antibodies and OC125 antibodies fo r detection, for interference due to antibodies induced by repeated ad ministration of F(ab')2 fragments of the anti-CA 125 antibody OC125. T esting 33 samples, obtained from 13 patients treated with OC125 fragme nts, we found falsely high CA 125 concentrations only in samples with exceptionally high concentrations of both anti-idiotypic antibodies an d non-specific human anti-mouse antibodies. In contrast, the recovery of added CA 125 was already diminished in the presence of low anti-idi otypic antibody concentrations. Both interferences disappeared after r emoval of serum IgG. It was possible to eliminate the falsely high res ults, but not the reduction in recovery rate, by adding non-specific m urine IgG. When the binding of the detector antibodies was performed i n a separate incubation step, no reduction in recovery rate was observ ed. Our results suggest, that non-specific human anti-mouse antibodies are responsible for falsely high results. The reduction in the recove ry rate is obviously due to an inhibition of the binding of OC125 dete ctor antibodies by anti-idiotypic antibodies. In patients receiving OC 125 antibodies CA 125 can be measured using OC125 detector antibodies if a two-step assay is performed. An increase in CA 125 following OC12 5 infusion should be confirmed after the addition of non-specific muri ne IgG.